Typically, landfill gas or biogas derived from a landfill comprises generally equally molal amounts of a mixture of carbon dioxide and methane, with the carbon dioxide and methane representing about 90 mole percent or more of the biogas. The landfill gas also contains minor amounts of nitrogen, oxygen, hydrogen, carbon monoxide and a variety of undesirable trade impurities present at the ppm level, as well as water vapor. The nitrogen and oxygen content of the biogas depends on the air ingress to the landfill- and gas-collection system. Generally, the landfill gas is extracted by employing a slight vacuum on a pipework manifold which is buried in the landfill. Landfill gas so extracted is then typically compressed and burned as a source of heat. Occasionally, such low BTU gas is used to fuel a generator for the direct production of electric power.
It is desirable to provide from the landfill gas methane of fuel-grade or sales-gas quality, typically with no more than 3 mole percent, or, more preferably, no more than 2 mole percent carbon dioxide content. Such high-grade fuel is readily marketable and widely useful. It is desirable also to obtain a high-purity carbon dioxide stream typically in liquid form, for sale at a purity of up to food-grade quality; that is, containing generally less than 10 parts per million of methane. A lower product quality may be acceptable, if the carbon dioxide liquid is to be employed as a refrigerating fluid or for well-field injection or other purposes. In these cases, a carbon dioxide product, containing up to 5 mole percent methane or other trace impurities, is typically usable.
The separation of the landfill gas into a liquid carbon dioxide product stream and a compressed fuel-quality methane product stream in an efficient and economical manner presents several problems. Landfill gas streams may be separated employing cryogenic fractionation in a cryogenic distillation column; however, in such cryogenic fractionation, while it can achieve a food-grade liquid carbon-dioxide product stream, the resulting methane stream contains at least 15 mole percent of carbon dioxide and is unsatisfactory for use as a sales-quality or fuel-grade gas stream without significant further processing.
The landfill gas stream also may be cryogenically fractionated employing the Ryan/Holmes process as described in U.S. Pat. No. 4,318,423, issued Mar. 9, 1982. In the Ryan/Holmes process, a first distillation column is employed to produce a methane stream as an overhead product, while carbon dioxide, plus ethane and heavier hydrocarbons, are produced as a bottom product stream. Since the carbon dioxide would normally freeze at the temperature encountered in a demethanizer, the Ryan/Holmes process employs a liquid additive agent, such as an alkane (like a C.sub.3 -C.sub.6) stream fed into the column which effectively prevents the freeze up of carbon dioxide in the column, so as to prevent acid gas solids, such as carbon dioxide, from occurring in the solids-potential zone of the column. However, the Ryan/Holmes process requires the introduction of an additive agent, and further results in the contamination of the bottom product stream containing the carbon dioxide with the additive agent, and, therefore, further processing steps are required, in order to recover carbon dioxide and to remove the additive agent.
The landfill gas stream may be treated in a multiple-stage gas-permeation membrane-type process or alternatively in a pressure-swing-adsorption process, to provide for the separation of the methane and carbon dioxide; however, such processes cannot achieve a high-purity liquid carbon dioxide product stream, while further the carbon dioxide product stream obtained is a vapor at a low pressure and would require significant further processing, to obtain a high-quality liquid carbon dioxide product stream.
The impurities or contaminants in a landfill gas may vary in type and amount, but typically contaminant moisture is generally at a saturated level, while nitrogen ranges from 0.5 to 4 percent; oxygen 0 to 1 percent; hydrocarbons (nonmethane) from about 500 to 4,000 ppm; halocarbons, oxygenated and sulfonated hydrocarbons from about 100 to 2,000 ppm; hydrogen sulfide from about 2 to 100 ppm; and carbon monoxide up to about 1,000 ppm. Various separate techniques and steps are employed to remove these trace contaminants from the landfill biogas; for example, but not limited to: for moisture, the use of refrigeration dryers, pressure-swing dryers, thermal-swing dryers and glycol scrubbers; for nitrogen, fractional distillation; and for oxygen, fractional distillation and deoxo units. For nonmethane hydrocarbons, activated carbon adsorption and catalytic oxidation are used, and for halocarbons, oxygenated and sulfonated hydrocarbons, activated carbon adsorption and catalytic oxidation are used. In the case of hydrogen sulfide, iron-sponge adsorption, impregnated-charcoal adsorption, and molecular-sieve adsorption techniques are used, and for carbon monoxide, generally catalytic oxidation.
Gas-permeation membrane apparatuses have been used in combination with a distillation column for the separation of an azeotropic mixture of carbon dioxide and ethane (see, for example, U.S. Pat. No. 4,374,657, issued Feb. 23, 1983). However, this process is directed to the particular process problems associated with carbon dioxide/ethane azeotropic feed streams, and does not provide for use of a landfill biogas or provide a high-purity carbon dioxide stream and a fuel-grade methane stream.
It is, therefore, desirable to provide a simple, effective and economical process for the separation of landfill biogas containing impurities into a high-quality carbon-dioxide vapor or liquid product stream and a fuel-grade compressed methane product stream.